1. Field of the Invention
The present invention relates to a piezoelectric transformer for use in an inverter for the backlight of a liquid crystal display, an inverter for starting a fluorescent tube, or a similar device.
2. Description of the Related Art
Conventionally, a Rosen-type piezoelectric transformer includes a rectangular, flat piezoelectric substrate, having a drive portion polarized in the thickness direction and a power generation portion polarized in the longitudinal direction. In order to achieve a reduction in size and a higher step-up voltage ratio, a monolithic piezoelectric transformer has been proposed. Piezoelectric ceramic sheets, each containing an internal electrode, are layered and fired into a single unit, thereby obtaining a monolithic piezoelectric transformer. FIG. 7 is a longitudinal sectional view of a conventional monolithic piezoelectric transformer.
The conventional piezoelectric transformer uses a primary (basic) mode or a secondary mode and includes a rectangular, flat piezoelectric substrate 1. A plurality of piezoelectric ceramic sheets 5 shown in FIG. 7 are layered and fired into a single unit, to thereby obtain the piezoelectric substrate 1. Pairs of input electrodes 21 and 22 are formed within and on the upper and lower surfaces of a longitudinal half portion (left-side half in FIG. 7) of the piezoelectric substrate 1 such that the input electrodes 21 and 22 face each other and extend over substantially the entire area of the longitudinal half portion of the substrate 1. An output electrode 3 is formed on the longitudinal end surface of the other longitudinal half portion of the substrate 1. The half portion including the input electrodes 21 and 22 functions as a drive portion A and is polarized in the thickness direction of the substrate. The other half portion including the output electrode 3 functions as a power generation portion B and is polarized longitudinally.
The input electrode 21 includes internal electrodes 21i formed within the piezoelectric substrate 1 and an external electrode 21e formed on the upper surface of the piezoelectric substrate 1 (as viewed in FIG. 7). The input electrode 22 includes internal electrodes 22i formed within the piezoelectric substrate 1 and an external electrode 22e formed on the lower surface of the piezoelectric substrate 1 (as viewed in FIG. 7). The electrodes 21i and 22i are formed alternately and between piezoelectric ceramic layers. The internal electrodes 21i and 22i have a length that is equal to that of the external electrodes 21e and 22e. The internal electrodes 21i are connected in common to the external electrode 21e, and the internal electrodes 22i are connected in common to the external electrode 22e.
An input line is connected to the input electrodes 21 and 22 at the node point thereof, whereas an output line is connected to the output electrode 3. An input voltage is applied to the piezoelectric transformer 1 through the input line and is stepped up by the piezoelectric effect and the reverse piezoelectric effect. The stepped-up voltage is obtained from the output electrode 3 through the output line.
A piezoelectric transformer having the above-described structure is used in a starting circuit for a cold-cathode tube serving as a backlight for a liquid crystal display. Light quality of the cold-cathode tube is adjusted by varying the output voltage of the piezoelectric transformer. The piezoelectric transformer is required to output a stable output voltage even when the input voltage varies. Usually, the output voltage is adjusted or regulated by varying a drive frequency for the piezoelectric transformer. Conventionally, the longitudinal dimension or length of the input electrodes (the length of the drive portion) is made longer than half the length of the piezoelectric substrate so as to make the frequency characteristic of a step-up ratio steep, thereby obtaining a desired output voltage without impairing efficiency. Such a piezoelectric transformer is proposed in, for example, Japanese Patent Application Laid-Open (kokai) No. 9-21362.
According to the above-described conventional monolithic piezoelectric transformer, the internal and external electrodes, which constitute the input electrodes, have the same length. Whenever the conditions of the step-up ratio and efficiency are to be modified by altering the length ratio between the drive portion and the power generation portion, jigs and tools for forming the external electrodes must be manufactured accordingly. Such jigs and tools include a screen mask for printing and a metallic mask for vapor deposition. Also, electrode patterns formed on a substrate on which the piezoelectric transformer is mounted must be modified accordingly.